Description of the Prior Art
Prior art optical character recognition systems incorporate means for automatically varying the discrimination reference level which a video pulse must exceed in order for the pulse to be recognized as a black or character pulse rather than as a white or document background pulse. For example, in one prior art single channel system, the areas of a moving document are time-sampled in series by a rotating slitted disc through which light passes to a photo-multiplier tube. The discrimination reference level for each video pulse is automatically adjusted by varying the cut-off level of a contrast tube in accordance with the greatest amplitude of recently sensed black video pulses.
In another example of the prior art, each video pulse corresponding to a document area being scanned by a two-dimensional array of photo cells is compared with the average value of the video pulses corresponding to areas surrounding the area being scanned, and a black or character recognition signal is generated only when the video pulse corresponding to the scanned area exceeds the average blackness of the surrounding areas.
Each of these prior art circuits occupy a large space and are not amenable for use in a hand-held optical character recognition device.